rh heads?

tyborg15 · Jun 9, 2013

Hi all,

I'm gradually getting all the goodies together for an engine rebuild and have been picking up on all the fascinating information available. One thing I cant find is - What does RH mean when you refer to the heads and is there anyone who can succinctly describe the different designations and flaws/benefits of each one.

Cheers

Tyborg

concours · Jun 9, 2013

This may help: http://atlanticgreen.com/nhth.htm

tyborg15 · Jun 9, 2013

Thanks Concours,

Informative article - I have the RH4.

concours · Jun 10, 2013

Thanks DynoDave!

grandpaul · Jun 10, 2013

Isn't RH4 the one that should be carefully inspected for cracks at the guide bores?

(or is that RH10?)

acotrel · Jun 10, 2013

I read that stuff on heads a while back, however I'd still like to know if the cylinder base gaskets, are the same on a 650, 750 and 850 Norton twin ? That makes it feasible to change both the head and barrels together, if the bolt patterns are the same.

JRD · Jun 10, 2013

850 barrels have the 4 allen bolts going through preventing to dismount head and barrels together.

concours · Jun 10, 2013

850 came with NO base gaskets.

lcrken · Jun 10, 2013

750 and 850 cylinder base bolt patterns are the same. The only change needed to put 850 top end on 750 is to remove the four base studs on the sides to allow the 850 through bolts to screw in. Also a good idea to fit thread inserts when converting to through bolts. 850 had helicoil inserts fitted from the factory.

Ken

kraakevik · Jun 10, 2013

I also thoroughly recommend helicoiling the four side base threads for the through-bolts--mechanics have been known to throw wrenches around when head bolts pull out just shy of the specified torque value.

When putting an 850 barrel and head on 750 cases it may well be necessary to open up the crankcase mouth by .040" or so to accommodate the bigger barrel. You might also consider looking for 850 pistons on the light side to keep the balance factor close to stock

Tim Kraakevik
kraakevik@voyager.net

dynodave · Jun 11, 2013

concours said:
850 came with NO base gaskets.

There IS a 850 base gasket. 06-3812
I keep a few in stock.

concours · Jun 11, 2013

dynodave said:
concours said:

850 came with NO base gaskets.

Click to expand...

There IS a 850 base gasket. 06-3812
I keep a few in stock.

So, which/when did they use them? My Cliffhanger ,er, uh, CLYMER :lol: manual said they were not used on 850 engine. (my '74 had no gasket when I found it) Used for compression decrease? Share with us... :mrgreen:

lcrken · Jun 11, 2013

Mk I and Mk II 850 did not have base gaskets, but had a plastic sealant ("Plastic Gasket", or something similar). The paper gasket was reintroduced on the Mk III. The 063812 part number is the Mk III base gasket.

Ken

lcrken · Jun 11, 2013

Like Tim said, the 750 crankcase will require opening up the case mouth. Also, you can use the 850 base gaskets on a 750, but not vice versa. The 750 gasket has a smaller opening for the cylinder protrusions.

According to John Hudson, the factory re-introduced the paper gasket because of concerns that over-enthusiastic application of plastic sealants by customers could block the oil drain passage.

Ken

79x100 · Jun 11, 2013

The trouble with using a base gasket on an 850 (and I do, 'cause the book said so) is that it's necessary to remove the head in order to check-tighten / re-torque the through bolts. In my experience, they do settle and are then effectively not clamping the barrel sides. This usually means the sacrifice of a head gasket.

I don't think that I'd want to delete the base gasket without first boring or removing any wear ridge as the top ring will otherwise be contacting the ridge.

Onder · Jun 11, 2013

You mean delete the base gasket, right?
Perhaps go with wellseal or similar and a silk or copper thread
and avoid the remove the head routine for retorque of the allens?
Pulling the head in the frame is less than fun.

79x100 · Jun 11, 2013

Onder said:
You mean delete the base gasket, right?

Indeed I do - I've edited the original post so that it makes sense - thanks!

I have pretty well decided that I'll just use sealant next time I'm putting on a re-bored cylinder. I wonder if the reason for the original non-use of a gasket on the 850s was because Norton feared the same problem, but then regarded it as a lesser evil than a blocked rear drain ?

concours · Jun 11, 2013

Onder said:
You mean delete the base gasket, right?
Perhaps go with wellseal or similar and a silk or copper thread
and avoid the remove the head routine for retorque of the allens?
Pulling the head in the frame is less than fun.

Loctite #518 is the weapon of choice for me. GREAT results on several engines, and HUNDREDS of industrial repairs.

rh heads?

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